Fixed linker warnings about overridden styleURL__

Replaced #3088 with a better fix that relies on Interface Builder’s lack of support for conditional compilation.

include/mbgl/ios/MGLMapView+IBAdditions.h

@@ -14,12 +14,16 @@ NS_ASSUME_NONNULL_BEGIN
 // inspectables declared in MGLMapView.h are always sorted before those in
 // MGLMapView+IBAdditions.h, due to ASCII sort order.
 
+#if TARGET_INTERFACE_BUILDER
+
 // HACK: We want this property to look like a URL bar in the Attributes
 // inspector, but just calling it styleURL would violate Cocoa naming
 // conventions and conflict with the existing NSURL property. Fortunately, IB
 // strips out the two underscores for display.
 @property (nonatomic, nullable) IBInspectable NSString *styleURL__;
 
+#endif // TARGET_INTERFACE_BUILDER
+
 // Convenience properties related to the initial viewport. These properties
 // are not meant to be used outside of Interface Builder. latitude and longitude
 // are backed by properties of type CLLocationDegrees, but these declarations

include/mbgl/ios/MGLMapView.h

@@ -234,9 +234,6 @@ IB_DESIGNABLE
 *   To display the default style, set this property to `nil`. */
 @property (nonatomic, null_resettable) NSURL *styleURL;
 
-/* Discourage programmatic usage of this IB-only property. Interface Builder skips over this declaration because it is unable to parse attributes. See the real declaration in MGLMapView+IBAdditions.h. */
-@property (nonatomic, nullable) IBInspectable NSString *styleURL__ __attribute__((unavailable("styleURL__ is for use within Interface Builder only. Use styleURL in code.")));
-
 /** Currently active style classes, represented as an array of string identifiers. */
 @property (nonatomic) NS_ARRAY_OF(NSString *) *styleClasses;